Pest regulation by natural enemies has a strong potential to reduce the use of synthetic pesticides in agroecosystems. However, the effective role of predation as an ecosystem service remains largely speculative, especially with minute organisms such as mites

Predatory mites are natural enemies for ectoparasites in livestock farms. We tested for an ecosystem-level control of the poultry pest Dermanyssus gallinae by other mites naturally present in manure in poultry farms, and investigated differences among farming practices (conventional, free-range and organic).

We used a multiscale approach involving (i) in-vitro behavioural predation experiments, (ii) arthropod inventories in henhouses with airborne DNA, (iii) a statistical model of covariations in mite abundances comparing farming practices.

Behavioural experiments revealed that three mites are prone to feed on D. gallinae. Accordingly, we observed covariations between the pest and these three taxa only, in airborne DNA at the henhouse level, and in mites sampled from manure. In most situations, covariations in abundances were high in magnitude and their sign was positive.

Predation on a pest happens naturally in livestock farms due to predatory mites. However, the complex dynamics of mite trophic network prevents the emergence of a consistent assemblage-level signal of predation. Based on these results, we suggest perspectives for mite-based pest control and warn against any possible disruption of ignored services through the application of veterinary drugs or pesticides.

The study was focused on the acarofauna of poultry manure. It was conducted in 20 barn henhouses (no cages, all birds on ground) distributed among 3 types of management practices (6 conventional, 8 free-range and 6 organic) and located half in eastern France down to Jura mounts (Ain region) and half in the South of the Rhône valley (Drôme region). The layout of French barn henhouses is similar in the three farm types. The inside always contains the basic equipment necessary for the hens and placed on the slatted floor (high stratum) under which manure accumulates (low stratum). We collected five standard manure samples from randomly-selected points under the slatted floor, during four sampling campaigns in each henhouse (n= 20 x 4 x 5), to characterize manure mite assemblages following the methodology in Roy et al. (2017).

Each sample consisted of 250 mL of manure taken from an area approximately 40 cm in diameter and 5 cm deep in a glass flask sealed with a rubber stopper. Exposure to saturating ethyl acetate vapours within one hour of sampling killed the arthropods and prevented further development of the assemblage. All arthropods from these samples were identified and counted using the Rapid Biodiversity Assessment method developed by Roy et al. (2017) for sorting out mite morphospecies and counting them using a binocular stereomicroscope.

Generally speaking, each flock (or batch of hens) is kept on the farm for one year (optimal laying period). At the end of this period, the hens are removed from the henhouse to make way for the next flock. For health reasons, a period of a few weeks' sanitary emptying, including massive cleaning, is systematically applied before the new flock enters the henhouse.

Among the farms included in the study, flock start-up dates were deliberately varied (desynchronized), and consequently, so were the dates of the emptying period. In order to obtain data on each of the 4 seasons, we conducted 5 successive campaigns, spaced exactly 3 months apart, and sampled each time in all the henhouses that had been in operation for more than 2 months (i.e. outside the emptying period or at least 2 months after the entry of the new flock). Indeed, a sufficient accumulation of manure is necessary for the development of the mite community, which colonizes the poultry houses from the outside. In this way, we obtained about 4 sampling groups per building, but not all of them successively. In addition, due to sanitary constraints, one of the buildings had to be excluded from the sampling campaigns after 2 campaigns (it was contaminated with salmonella after the second campaign and we had committed ourselves to abandon sampling in the infected buildings in order to avoid spreading the pathogens).